In common practice, when it is desired to produce hydrocarbons from a subsurface formation, a well is drilled from the surface until it intersects the desired formation. As shown in FIG. 1, a typical drilling operation entails a surface operating system 50, a work string 100 that may comprise coiled tubing or assembled lengths of conventional drill pipe, and a bottom hole assembly (BHA) 200. Surface system 50 typically includes a drilling rig 10 at the surface 12 of a well, supporting drill string 100. BHA 200 is attached to the lowermost end of work string 100. Operating system 50 is positioned at the surface adjacent to well 12 and generally includes a well head disposed atop of a well bore 18 that extends downwardly into the earthen formation 20. Borehole 18 extends from surface 16 to borehole bottom 30 and may include casing 22 in its upper zones.
The productivity of formations can vary greatly, both vertically and horizontally. For example, in FIG. 1, formation 21 may be a producing formation (stratum), while formation 20 above it may be a non-producing formation. The target formation(s) have typically been mapped using various techniques prior to commencement of drilling operations and an objective of the drilling operation is to guide the drill bit so that it remains in the target formation. Thus, in many wells, the lower portion of the borehole deviates from the vertical and may even attain a substantially horizontal direction. In these circumstances, it is desirable to drill the well such that borehole 18 stays within the producing formation 21.
Similarly, it is sometimes desired to guide the drilling of a well such that it parallels another well. This is the case in steam-assisted gravity drainage (SAGD) drilling, in which steam injected through one of a pair of parallel wells warms the formation in the vicinity of the wells, lowering the viscosity of the formation fluids and allowing them to drain into the second well. The second well thus functions as a production well and typically is drilled such that it lies below the injection well.
As a result of this deviated, directional, or horizontal drilling, the drill bit may traverse a sizable lateral distance between the wellhead and the borehole bottom. For this reason, and because the degree of curvature of the borehole is often not known precisely, it also becomes difficult to know the true vertical depth of the borehole bottom. Hence, it is preferred to track the position of the bit as precisely as possible in order to increase the likelihood of successfully penetrating the target formation.
It is particularly desirable to accurately locate the position of the bottom hole assembly (BHA) during drilling so that corrections can be made while drilling is ongoing. Determining the precise location of the drill bit as it progresses through the formation and communication of that information from the downhole location to the surface are two significant problems that have not heretobefore been adequately addressed. Both objectives are made more difficult by the drilling operation itself, which involves at least rapid fluid flow, moving parts, and vibrations.
Various methods are traditionally combined to achieve these goals. Gyroscopes and various types of sensors have been used to track bit movement and/or bit position. Electromagnetic (EM) telemetry is one technique used for transmitting information, either to the surface or to another location uphole. Other transmission techniques involve mud pulses or acoustic signaling using the drillstring as the signal carrier. Current techniques are not very accurate or rapid, however, and can result in erroneous calculations of the position of the BHA. Hence, it is desirable to provide a technique for determining the position of a bit in a subterranean formation that eliminates or at least substantially reduce the problems, limitations and disadvantages commonly associated with the known bit-tracking techniques.